1. Technical Field
The present invention relates to an injection molding apparatus and an injection molding method, in particular, to those suitable for producing a disc substrate.
2. Background Art
As a conventional molding method for optical discs, such as compact discs (CDs) and digital versatile discs (DVDs), the injection molding method has been used. In the injection molding method, with a pair of a fixed die and a movable die, a space is formed. The space is called a cavity. Molten resin material is injected in the cavity by an injection nozzle through a sprue bush. Therefore, the injected resin is cooled and solidified in the dies. As a result, a disc substrate is molded.
FIG. 1 is an outlined sectional view showing an example of a tip portion of an injection nozzle of conventional dies and a peripheral portion thereof. A nozzle 101 has a cylindrical shape. A tip portion of the nozzle 101 has a semi-spherically protruded surface. An internal space 103 of the nozzle 101 and the outside of the nozzle 101 are connected through a resin injection hole 104 at the tip portion of the nozzle 101. Another tip portion, not shown, of the nozzle 101 is, for example, thread-connected to a cylinder, not shown. In the cylinder, a heater that melts the resin material, a screw that pushes the molten resin material out of the cylinder to the internal space 103, and so forth are disposed. In such a structure, the molten resin material can be injected out of the resin injection hole 104 to the outside of the nozzle 101.
The die side in which the resin material is injected comprises a fixed die 107 and a movable die 108. When the fixed die 107 and the movable die 108 are set, a cavity 109 is formed. A sprue bush 102 is engaged with the cavity 109 of the fixed die 107 so that the sprue bush 102 is positioned perpendicular to a main plane that forms the cavity 109 of the fixed die 107. The sprue bush 102 is formed in a cylindrical shape and has an axial resin flow path 105. The resin flow path 105 allows the molten resin material supplied from the nozzle 101 to flow in the cavity 109.
The sprue bush 102 has a portion that contacts the tip of the nozzle 101. The portion is formed in a semi-spherically recessed shape. The radius of curvature is larger than the radius of curvature of the semi-spherically protruded surface of the tip of the nozzle 101. The difference of the radius of curvature of the semi-spherically recessed surface of the sprue bush 102 and the radius of curvature of the semi-spherically protruded shape of the nozzle 101 helps to press-contact the nozzle 101 and the sprue bush 102 without a space and to align easily the resin injection hole 104 and the resin flow path 105.
In addition, the sprue bush 102 has an inner ring-shaped coolant flow path 106 around the resin flow path 105. The coolant flow path 106 allows coolant supplied from the outside to flow so as to cool the sprue bush 102. In reality, cooling water that flows in the coolant flow path 106 allows the temperature of the sprue bush 102 to lower and thereby solidify the molten resin. The fixed die 107 and the movable die 108 have respective cooling units, not shown, along with the coolant flow path 106.
Next, with reference to FIG. 1, a molding method for a disc substrate by a conventional injection molding apparatus will be described. As shown in FIG. 1, the nozzle 101 is press-contacted to the sprue bush 102. The movable die 108 and the fixed die 107 are set. As a result, the cavity 109 is formed. In this state, the molten resin material for one shot is pushed from the supply side of the resin material, namely the cylinder side, not shown, toward the tip portion of the nozzle 101. Thus, the molten resin material is injected into the cavity 109 through the resin flow path 105 of the sprue bush 102.
Thereafter, both the fixed die 107 and the movable die 108 are cooled. In addition, the coolant that flows in the coolant flow path 106 cools the molten resin material. Thereafter, an ejector pin, not shown, separates a sprue portion from the resin material in the cavity 109. As a result, a disc substrate shaped by the cavity 109 is molded. The sprue portion is the resin material at a center hole portion formed at a center portion of the disc substrate. The resin flow path 105 causes resin material at the center hole portion to form a rod-shaped protrusion portion, so-called sprue, at the center portion.
After the resin material has been solidified, the movable die 108 is separated from the fixed die 107 and the molded disc substrate and sprue portion are removed from the fixed die by a removing unit. As a result, the disc substrate is molded by the injection molding apparatus.
However, when the foregoing injection molding apparatus molds a disc substrate, the following problem arises. In other words, when the sprue portion is removed from the fixed die by the removing unit, thread-shaped resin, so-called stringiness, may take place at a tip of the rod-shaped sprue protrusion portion. It is taught that stringiness occurs because the temperature of the resin material in the tip portion of the nozzle 101 does not sufficiently lower. Thus, to shorten the molding time for a disc substrate, in particular, if the cooling time for which the molten resin material is solidified is shortened, stringiness unavoidably occurs.
If the stringiness takes palace, string-shaped resin that was cut in this shot is molded in the next shot. As a result, since a string-shaped stripe would remain in the appearance of the molded product, it is treated as a defective product. In addition, the string-shaped resin would get caught in the removing unit. As a result, a sprue removing defect would take place.
To prevent stringiness from occurring, the setting temperatures of the nozzle 101 and the sprue bush 102 would be remarkably decreased. In this case, since the flow resistance of the resin material increases, the molding condition of the disc substrate would deteriorate. As a result, a good molded product would not be obtained.
Therefore, an object of the present invention is to provide an injection molding apparatus and an injection molding method that allow the temperature of a nozzle tip to lower so as to prevent stringiness from occurring and a good product, for example, a disc substrate, to be molded.